Hull inspection has become increasingly important in recent years, particularly for larger vessels. Visual inspection by divers is a very undesirable prospect and can only be considered in clear waters where visibility is good enough for close inspection. Under-hull working removes the diver's direct escape route to the surface, which is fundamental for safe subsea working.
Remotely Operated Vehicles (ROVs) have been used for hull inspection using video imaging or short range, high resolution, sonar for inspection. ROVs are typically operated from the surface along side the ship and require connection via control wires for control and relay of inspection data. A ship's hull presents many opportunities for control wire snagging and the surface connection restricts manoeuvrability. Wireless communication of control and inspection data with a subsea vehicle would prove particularly advantageous in this application.
To date, acoustic modems have been the dominant choice for underwater wireless communications. However, essentially horizontal communication in the restricted space below a ship's hull is a particularly challenging application for acoustics. For example, in a typical quayside inspection berth there is often less than 10 m clearance between the ship's hull and the seabed. The narrow channel formed by ship's hull on the upper boundary and sea bed in lower boundary, typically with a vertical quayside enclosing a third boundary, creates a severe multi-path environment which makes acoustic communications impractical.
Navigation presents a further problem for hull inspection systems. Inspection data must be accurately associated with its corresponding location. Divers can quickly become disorientated when presented with the uniform inverted surface of a ship's hull and have no accurate frame of reference for relocating interesting features that are identified during inspection. Sonar navigation methods have proven impractical due to the multi-path acoustic environment as outlined above.